The invention is in the field of liquid-crystalline polyesters suitable for use in optical data processing, more particularly liquid-crystalline polyesters which are infrared-sensitive due to the presence of an infrared-absorbing dye.
Liquid-crystalline polymeric compositions containing infrared-absorbing dye are known from the literature. Such polymeric compositions, including polyesters, are described in European Patent Application 0 302 497. Information can be written in such polymeric compositions with the use of a solid-state laser (.lambda.=750 to 870 nanometers). The infrared-absorbing dye serves to convert the laser light energy into thermal energy. According to the above-mentioned publication, information is written in films of liquid-crystalline polymers in the following manner. The film has been aligned in advance with, for instance, an electric field, and so consists of one domain and is hence transparent. Using a laser, the film is heated locally to above a phase transition temperature and then rapidly cooled. As a result of the rapid cooling the heated section of the liquid-crystalline layer is not restored to its original transparent state but remains light-scattering, i.e., opaque. In this way opaque lines can be written in a transparent film.
Writing transparent lines in an opaque film is also known from other publications. In those cases an unaligned film is employed, which is heated locally with a laser. During the irradiation with the laser an electric or magnetic field is applied, the heated section of the film thus being aligned and hence rendered transparent. (Cf., e.g., GB-A-2 146 787).
For writing data it is desirable to have the highest possible concentration of dye in the polymeric composition without, of course, said composition's properties being adversely affected. The drawback to the above-mentioned polymeric composition is that the infrared-absorbing dye is mixed in with the polymer, to form so-called solid solutions. As a rule only low concentrations of dye can be dissolved in these polymers, since otherwise there will be crystallization or segregation of the dyes, especially on the long term. Moreover, in the case of solid solution polymers it is often hard to make optically transparent films with homogeneous dye distribution.
Liquid-crystal polymers are also known in which the dye is incorporated into the polymer chain. For instance, in U.S. Pat. No. 4,896,292 dye is incorporated into the polymer chain to adapt the absorption behavior of liquid-crystalline polymers to the wavelength of the laser to be used. However, the dyes in question are not infrared-absorbing dyes. These known dyes (.lambda.=633 nm) generally are much more stable than infrared-absorbing dyes, but they are insensitive in the wave-length field of solid-state lasers (.lambda.=750-870 nm).
If it is desired to utilize solid-state lasers, infrared-absorbing dyes have to be added to the polymer. The advantage of solid-state lasers over other lasers is that they are small in size and relatively inexpensive.
Further, so-called polymeric dyes, which absorb in the infrared wavelength field, are known. However, these polymers are not liquid- crystalline. For use in optical data storage a thin layer of such a polymeric dye is applied to a film of liquid-crystalline polymer. This is, however, not an optimum solution to the problem either. The adhesion between the layers and the sensitivity of the resulting film generate a score of difficulties.
To attain the object of the invention, an infrared-sensitive liquid-crystalline polymeric composition suitable for optical data storage in which the dyes are homogeneously distributed and do not segregate or crystallize out, and which has sufficient sensitivity, two problems need to be solved.
First, liquid-crystalline polymeric compositions suitable for optical data storage must be found into which infra-red absorbing dyes can be incorporated without the liquid-crystalline behavior of the polymeric composition being adversely affected.
Secondly, infrared-absorbing dyes must be found which are stable enough for incorporation into liquid-crystalline polymers and also possess good thermal and photochemical stability. Moreover, the dye must be functionalizable, i.e., it must be possible to render the dye co-polymerizable, e.g. esterifiable.